1. Field of the Invention
The present invention relates to a temperature sensor with leads, and more specifically, to a temperature sensor with leads suited for measuring a temperature at a location remote from a control board on which a temperature sensor is mounted.
2. Description of the Related Art
A personal computer or a power assisted bicycle may use a relatively large secondary battery or a plurality of fuel cells arranged in parallel in some cases. In such cases, because a location where a temperature of a secondary battery or fuel cells is to be measured is remote from a control board on which a temperature sensor is mounted, a temperature sensor with leads utilizing a long lead is employed. The use of such a temperature sensor with leads enables the leads to be fixed on the control board (e.g., a printed circuit) by soldering and also allows a temperature sensing portion of the temperature sensor to be arranged adjacent to a part whose temperature is to be measured. Therefore, the temperature of the location where the temperature is to be measured can be detected with high precision.
One example of this type of a temperature sensor with leads is illustrated in FIG. 21 (Japanese Unexamined Patent Application Publication No. 10-149903). The temperature sensor includes a single plate thermistor 103 having a plate thermistor body 101 having both surfaces on which electrode layers 102a and 102b are formed. The thermistor 103 is sandwiched between leads 105a and 105b each having a tip to which a heat-resistant conductive paste 104 containing a glass frit is applied. An insulating inorganic substance layer 106 is formed on the surface of the thermistor element 101. These components are disposed in a glass tube 107 such that the connection part is covered. The electrodes are baked, and the components are simultaneously glass-sealed.
Another related-art example is illustrated in FIG. 22 (Japanese Unexamined Patent Application Publication No. 11-108771). This temperature sensor includes a pair of leads 112a and 112b drawn out of a thermistor 111, and the lead 112a is U-shaped. The thermistor 111 and insulated leads 113a and 113b arranged substantially in parallel are connected through solder 114a and 114b. The thermistor 111, the solder 114a and 114b, and the tip of each of the insulated leads 113a and 113b are housed in a heat-shrinkable insulator 115 made of, for example, polyolefin resin.
The temperature sensor described in Japanese Unexamined Patent Application Publication No. 11-108771 is designed to have a fixed gap (e.g., approximately 5 mm to 10 mm) in the longitudinal direction between the solder 114a and 114b such that they are not in contact with each other.
When the structure in which the plate thermistor 103 is sandwiched between the pair of leads 105a and 105b is used, as in Japanese Unexamined Patent Application Publication No. 10-149903, if an excessive thermal stress is applied, the insulating member coating each of the metal lines is thermally expanded, and additionally, a stress is produced in a direction in which the gap between the leads 105a and 105b broadens by the difference in thermal expansion between the leads 105a and 105b and the glass tube. That is, if an excessive thermal stress is applied, a stress occurs in the leads 105a and 105b in a direction in which the gap therebetween extends outwardly, as indicated by the arrows “a.” Because of this, a mechanical stress occurs in the electrode layers 102a and 102b in a direction in which they become separated from the thermistor body 101, as indicated by the arrows “b.” The glass tube may be broken.
In particular, if the leads 105a and 105b have different lengths or if there is a temperature difference between the leads 105a and 105b, because the amount of thermal expansion is also different, the stress indicated by the arrows b occurs.
As a result, a part of the electrode layers 102a and 102b may become separated from the thermistor body 101, this may increase the value of resistance, and thus a problem arises in which a measurement error is present between a measured temperature and an actual one. The stress indicated by the arrows a and b caused by an excessive thermal stress may break the thermistor body 101 itself, and the breakage may result in reduced reliability.
Additionally, because the thermistor 103 is sandwiched between the leads 105a and 105b, the dimension in the width direction is restricted by the thickness of the thermistor. This reduces the distance between the electrodes and increases the risk of a short circuit. Accordingly, there is also a problem in which it is difficult to further reduce the size.
In the temperature sensor described in Japanese Unexamined Patent Application Publication No. 11-108771, because the thermistor 111 is arranged such that the longitudinal direction of the thermistor 111 and the insulated lead 113b are parallel with each other, it can be considered that almost no stress occurs in a direction in which the electrodes become separated.
However, for the temperature sensor described in Japanese Unexamined Patent Application Publication No. 11-108771, although the solder 114a and solder 114b is designed to have a fixed gap in the longitudinal direction such that they are not in contact with each other, because of the structure in which the leads 112a and 112b are arranged in parallel in the longitudinal direction of the thermistor 111, an externally applied shock may separate the solder 114a and solder 114b. In addition, a stress is likely to be focused on the connection part between the leads 112a and 112b and the thermistor 111, so electric connection may be unstable. Because of this, for the temperature sensor described in Japanese Unexamined Patent Application Publication No. 11-108771, it is essential to cover the thermistor 111 and the leads 112a and 112b with the heat-shrinkable insulator 115. That is, for the type of the temperature sensor described in Japanese Unexamined Patent Application Publication No. 11-108771, it is necessary to cover a spatially large area extending from the thermistor 111 to the leads 112a and 112b with the heat-shrinkable insulator 115.
However, because the insulating resin forming the heat-shrinkable insulator 115 has low thermal conductivity, if the heat-shrinkable insulator 115 covers that area, the detection sensitivity is lower than that occurring when the heat-shrinkable insulator 115 is not provided.